A portable global optimizer and linker

To reduce complexity and simplify their implementation, most compilers are organized as a set of passes or phases. Each phase performs a particular piece of the compilation process. In an optimizing compiler, the assignment of function and order of application of the phases is a critical part of the design. A particularly difficult problem is the arrangement of the code generation and optimization phases so as to avoid phase ordering problems caused by the interaction of the phases. In this paper, we discuss the implementation of a compiler/linker that has been designed to avoid these problems. The key aspect of this design is that the synthesis phases of the compiler and the system linker share the same intermediate program representation. This results in two benefits. It permits the synthesis phases of the compiler to be performed in any order and repeatedly, thus eliminating potential phase ordering problems. Second, it permits code selection to be invoked at any point during the synthesis phases as well as at link time. The ability to perform code selection at link time presents many opportunities for additional optimizations. Measurements about the effectiveness of using this approach in a C compiler on two different machines are presented.